This invention relates to a process for the degassing of liquids, especially polymer melts and solutions, e.g. for the continuous processing of thermoplastic polymers, and the invention also relates to an apparatus for carrying out the process.
Some thermoplastic polymers exhibit certain proportions of monomers, oligomeric solvents and other low-boiling liquid components as undesirable residues of their method of production. Other thermoplastics, such as polyamides, polymethylmethacrylates, polycarbonates and so-called "filled" polyolefines, etc., are very hygroscopic so that they have a strong tendency to take up moisture. The acquired moisture can be sealed into the interior of the product or adsorbed as a surface skin or it can be layered in the intermediate regions of the treated product. Moreover, in the extrusion of pulverulent molding batches, a high proportion of air is enclosed. Monomers (e.g. styrene) and solvents (e.g. ethylbenzene) often give the finished product an unpleasant odor. Besides, there are health considerations since many of these residual materials are toxic. Moisture causes the extruded semifinished product to exhibit a poor appearance and to possess mechanical properties which do not meet standard requirements. These unsatisfactory results occur because moisture and/or low boiling liquids, on account of the high processing temperatures of the raw material, are transformed into the gaseous state and lead to bubbles and striations which are outwardly visible in the finished product. A further reason is that some plastics tend to chemically decompose in the presence of moisture and/or the use of high temperatures, based upon suitable residence times.
In order to manufacture products with optimum qualities and properties, it is essential to remove the harmful liquid or gas components by degassing before or during the extrusion process. The degassing of the molding batch can be achieved by heating the material and by lowering the partial pressure in the space over the material or by simply evacuating this space.
For the degassing of polymer melts, it has been taught to install multiple screw extruders. See, for example, the so-called two stage cascade extruder disclosed in British patent specification No. 1,375,237 (1974), using a vacuum chamber between two screw extruders but avoiding a polymer melt in the first stage extruder. One problem with these measures resides in the fact that the capacity of the first screw cross section and the degassing region must be so coordinated with each other, that on the one hand an optimum throughput is achieved but, on the other hand, if too much melt is delivered from the first screw cross section, the degassing zone can become filled with melt and melt can be suctioned away.
Devices are known which avoid these particular disadvantages, but they require substantially higher priced equipment, e.g. shiftable or adjustable screw conveyors. A further disadvantage in the use of various screw extruders is the formation of deposits in the partially filled degassing zone of the screw and longer residence times of the melt in the extruder, leading to interruptions in production and a severe reduction in the quality of the product. Frequently also, problems arise if a higher melt pressure is permitted to build up with degassing extruders in order to overcome the pressure loss in the following apparatus, e.g. melt conduits, filters and nozzles. For the solution of this problem, in part, one can install very long extruders or one can connect melt pumps to the extruder. Both solutions are associated with high costs and lead to an additional thermal loading or stress of the product.